BOTHELL, Wash. – Seattle Genetics (SGEN) has halted all clinical trials of its vadastuximab talirine due to higher incidents of death associated with the treatment for acute myeloid leukemia (AML) patients. This is the second time the treatment has been halted due to fatalities.

The news caused shares of Seattle Genetics to fall more than 11 percent in premarket trading to $57.25, down from $64.52.

This morning, Seattle Genetics said the decision to halt the anticipated Phase III Cascade trial came after consulting with the Independent Monitoring Committee and examining unblended trial data that showed a higher rate of death, including fatal infection, in the vadastuximab talirine trial arm. As a result, Seattle Genetics said it was suspending all clinical trials associated with vadastuximab talirine, including the ongoing phase I/II clinical trial in frontline high risk myelodysplastic syndrome (MDS).

Vadastuximab talirine (SGN-CD33A; 33A) is a novel investigational ADC (antibody drug conjugate) targeted to CD33. An antibody-drug conjugate (ADC), it is designed to be stable in the bloodstream.

Clay Siegall, president and chief executive officer of Seattle Genetics, said the data was both unexpected and disappointing. Siegall said the company will closely review the data and consult with the U.S. Food and Drug Administration regarding the potential future of vadastuximab talirine.

Seattle Genetics’ decision came only a few months after the U.S. Food and Drug Administration lifted a hold on two Phase I clinical trials for vadastuximab talirine. The FDA slapped a hold on that trial in December after four patients died. In total six patients in that trial were shown to have hepatotoxicity, including several cases of veno-occlusive disease. Two additional Phase I trials also received a partial hold at the time. The clinical hold was lifted in March following an intense examination of trial data and the establishment of new safety protocol amendments. While the company resumed the two Phase I trials, Seattle Genetics opted to terminate a Phase I/II trial of vadastuximab talirine monotherapy in pre- and post-allogeneic transplant AML patients due to “he challenges of developing therapies in this specific setting.”

Siegall tried to put a positive spin on the news by touting other drugs in the company’s pipeline. Siegall said the company is looking to a Phase III trial of Adcetris as a frontline Hodgkin lymphoma therapy. He also said the company is “on track” to advance enfortumab vedotin into a pivotal trial in metastatic urothelial cancer in the second half of 2017 under Seattle Genetics’ collaborative deal with Astellas.

The halting of the vadastuximab talirine trials will certainly be a blow to the company that is still trying to shake off the crashing of a $2 billion deal with Immunomedics (IMMU). The two companies had planned to develop IMMU-132, Immunomedics’ proprietary solid tumor therapy candidate, but that deal was challenged by Immunomedics’ investors who said the deal was not good for the company.